As a possible battery for diverse applications due to its high energy density, the lithium-air battery has been recently capturing increasing worldwide attention. The most important component involved in this battery system is the air diffusion electrode. The properties of the air electrode determine directly the performance of the whole battery. The significant variables of the air electrode, which are critical for its properties, include the surface area, porosity, thickness, catalysts, conductivity as well as polarity for various organic electrolytes. Among these factors, catalysts for oxygen electrochemical reduction not only enhance the discharge properties of the lithium-air battery, but also reduce overvoltage during the discharge. Therefore, energy and power densities are improved.
The oxygen electrochemical reduction mechanisms for lithium-air batteries in non-aqueous liquid electrolyte is a mixed reaction mechanism, which involves a one-electron oxygen reduction reaction Li++O2+e−→LiO2, two two-electron oxygen reduction reactions 2Li+O2+2e−→Li2O2 and Li2O2+2Li++2e−→2Li2O, as well as a chemical transfer reaction 2LiO2→Li2O2+O2. Lithium-air batteries operated in a different electrolyte may involve a different reaction process; and even for batteries operated under the same conditions the reaction process may depend on the setup voltage. Such complexities of the oxygen electrochemical reduction reaction in the lithium-air battery lead to some uncertainty in the selection of air electrode materials, electrolytes including solvents and conductive support salts. Although the non-aqueous lithium-air battery offers the exciting possibility of substantially higher capacity, much work remains to be done in order to improve its performance, such as lowering the discharge overvoltage and discharge capacity, the low practical energy density, low power density, poor cyclability, and (air) humidity issue (anode stability).
The behavior of the lithium-air battery involves the oxygen reduction reaction dynamics itself and active species diffusion, i.e., diffusion of oxygen from outside into inside of the air electrode, and movement of the anode reaction product Li+ ions from the anode surface to the air electrode through the electrolyte. The diffusion of active species is related to the physical characteristics of the air electrode and electrolyte. To improve the capacity of the lithium-air battery, various approaches have been reported, for example the use of a bi-layer carbon electrode consisting of an active layer and a diffusion layer, different carbon materials, transition metal platinum and gold particles loaded onto the carbon, polymer-metal composites, different conductive electrolytes, as well as protected anode and catalyst materials. The intrinsic reaction of oxygen reduction was generally accelerated by the use of catalysts.
Carbon nanotube and nanofiber film-based materials, sometimes referred to as “Buckypaper,” are known for use in polymer exchange membrane fuel cells (PEMFCs) because carbon nanomaterials typically exhibit high conductivity and large specific areas, relatively low microporosity and good resistance to electrochemical corrosion. The use of such materials in membrane electrode assemblies of fuel cells is described in U.S. Patent Application Publication US 2010/0143822 published Jun. 10, 2010, the disclosure of which is incorporated fully by reference. See also J. P. Zheng, R. Y. Liang, M. Hendrickson, and E. J. Plichta, J. Electrochem. Soc. 155, A432-A437 (2008) and J. P. Zheng, P. Andrei, M. Hendrickson, and E. J. Plichta, J. Electrochem. Soc. 158, A43 (2011), the disclosures of which are incorporated by reference.